It is often desirable to regulate the temperature of a heating or cooling system. In this respect, controllers and control systems are commonly used. These controllers and controls systems help obtain, maintain, or change the temperature of the system. Typically, controllers for heating or cooling systems are electric in nature. These controllers are termed “electric” because they function by regulating or modulating some electrical aspect of the system, such as the system voltage, or the system power.
Thermostatic and steady-state electric controllers are among the most common types of controllers for thermoelectric module (“TEM”) based heating or cooling systems. Compared to compressor driven refrigerators and resistive electric heaters, TEM based systems typically operate at higher currents and on DC rather than AC current. This has heretofore made traditional low-cost controllers such as bimetal thermostats, unacceptable for use as controllers for TEM based systems.
A thermostatic controller operates by maintaining a temperature between two temperature limits. That is, a thermostatic controller operates to control the temperature of a cooling system by turning on or off cooling power when certain temperatures are reached. For example, when the temperature of the system gets too high, the controller turns on the cooling power to cool the system down. When the lower temperature limit is reached, the cooling power is turned off, and this cycle repeats itself to maintain the system temperature within the upper and lower temperature limits. The difference between the two set temperature limits is known as the system's hysteresis.
A steady-state controller, on the other hand, is designed to continually hold a set-point temperature with very little variation. It is often the controller of choice when a system temperature must be maintained with a high degree of certainty. When the steady-state temperature is disrupted, (e.g., by a change in ambient conditions) the controller acts to quickly bring the temperature back to the steady-state temperature. Steady-state control is often achieved with some variant of a proportional controller.
Electromechanical devices such as bimetal snap disks or relays are typically not used to control the temperature of TEM based systems. This is because direct current switching leads to contact pitting and premature wear from arcing, and because the number of switching cycles of the mechanical component limits the life of the device. In addition, the hysteresis of an electromechanical system is often set undesirably large in order to avoid premature device failure. Furthermore, snap disks are difficult to incorporate into an adjustable set-point device. This has lead to an almost uniform adoption of electric controllers as necessary components of TEM based systems. Some devices employ an electric controller plus additional structural components for altering between heating and cooling modes. The selection of a suitable controller is often one of the biggest considerations when designing heating or cooling systems, especially since electrical controllers have proven to be very costly.
Accordingly, improved controllers and control systems capable of regulating the temperature of TEM based heating or cooling systems would be desirable.